High performance composite structures such as are used in forming surface skin components of aircraft and space vehicles usually are formulated of fiber reinforced plastic compositions. Such materials include glass fibers such as E-glass or S-glass fibers, boron or carbon fibers, and aramid fibers such as the products identified as Kevlar fibers. The plastic matrix materials useful in the formation of such composites include polyester and epoxy resins, polyimides, polyamides, polybutadiene resins and vinyl ester polymers. Such materials can be cured by cross linking at temperatures ranging from room temperature up to about 400.degree. F. to 600.degree. F. or by application of chemical cross linking agents. Where the structures are to be subjected to high temperature conditions as in the case of radomes for high performance aircraft and heat shields for space vehicles, thermosetting resins such as those described above usually will be employed in the matrix material. In other cases, where less severe conditions are to be encountered, thermoplastic resins can be employed as the matrix material although in most cases thermoset resins are used.
In forming products having relatively simple shapes such as flat sheets or rectangular polyhedra, layup and bagging techniques can be prepared with a fair degree of success. For example, Kirk-Othmer, "Encyclopedia of Chemical Technology," 3rd Edition, 1981, Supplement Volume at pages 268-270, discloses a process for preparing a flat composite product by placing a carbon-fiber-epoxy prepreg layup on a flat tool surface. The layup is covered with breather plies and a nylon bag is placed over the breather plies and sealed at its edges to the tool surface. A vacuum is pulled upon the sealed assembly in order to evacuate air from the layup. The assembly is placed in an autoclave, heated and pressurized to effect a cure cycle.
A similar approach has been proposed for use in molding three dimensional composite structures. For example, as disclosed in U.S. Pat. No. 3,962,394 to Hall, a cylindrical or rectangular tubular mandrel is coated with a resin fiber layer which is surrounded by a compression sleeve formed of a thin film of nylon or rubber which is perforated with holes and split lengthwise. A layer of absorbent material is placed around the split compression sleeve and this assembly is surrounded by a plastic bag or bladder which is sealed at both ends to the tubular mandrel. The bladder is evacuated in order to cause the compression sleeve to compact the layers and expel trapped air and excess resin from the fiber-resin material through the holes in the compression sleeve.
Typically, in forming plastic composite products 5-10, sometimes more, plies of fiber mats are integrated together with intervening applications of resinous material matrix material to arrive at the final product. Each fiber ply comprise fibers which are oriented generally in a two dimensional surface of the ply, e.g., in a plane in the case of a flat composite product or in the wall of a cylinder in the case of a cylindrical product. The fiber ply may be of a continuous strand type as in the case of a filament wound on a molding tool. Alternatively, the fiber material may be formed of a plurality of generally parallel continuous fibers, e.g. in the form of an "unidirectional tape", or it may comprise chopped fibers aligned in an unidirectional manner. Such chopped fibers typically are of a length within the range of 0.1-3 centimeters.
In addition, the fiber mat may take the form of a braided structure in which the fibers extend predominantly along one direction but are braided or woven together, normally to provide an angle between strands, the "braid angle" of about 15.degree.-45.degree.. A fabric type structure in which fibers are interconnected by cross-strands intersecting at about 90.degree. may also be employed. The fiber layers may be in the form of prepregs in which the fibers are impregnated with an uncured resin which is later crosslinked in order to provide matrix material.
As described in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, Vol. 13, pp. 968-978 under the heading "Laminated and Reinforced Plastics" and in the Supplement Volume, pp. 260-281, under the heading "Composites, High Performance," various procedures are available for forming fiber reinforced plastic composites. A basic approach involves a technique in which fiber layers, which may or may not be prepregs containing resin, are disposed on a forming tool and resin, usually an uncured thermosetting resin, is either sprayed or painted on the fiber layers. Additional fiber layers and resin layers are added until the desired thickness is achieved and the resulting lay-up is then cured to produce the final product. The lay-up structure can be squeezed together under a light force in order to force the resin and fibers into intimate contact. Curing can take place under an applied pressure. Other processes useful for forming cylindrical products involve the winding of a fiber filament around an internal mandrel. The filament is wound onto a rotating mandrel and resinous matrix material applied, either by running the filament through a tank of uncured liquid resin or by spraying or painting the liquid onto the fiber filament as it is disposed on the mandrel.
In some composite products, it is desirable that the composite fiber plies be in close juxtaposition to one another with the resin material intimately mixed therewith. In other procedures, pronounced layering or lamination occurs. For example, U.S. Pat. No. 4,269,884 to Dellavecchia et al discloses a process of forming a stampable thermoplastic sheet which comprises several more or less discrete layers. In the Dellavecchia procedure, outer layers are formed of a thermoplastic resin which may optionally contain up to 50% of a particular filler and up to 45% of nonsiliceous fibers having a length ranging from about 0.01 to 3/4 of an inch. The fibers are generally oriented two dimensionally in a plane parallel to the plane of the sheet. Inside of the outer layer another resinous sheet is provided. This is in a molten state during the processing procedure to allow the internal fiber mats to be impregnated by the resin. Fiber mats are disposed upon an internal supporting screen. The fiber and resin layers are passed through rollers which apply a pressure to the sheets of between 1000 to 1500 pounds per linear inch to ensure bonding of the several layers and impregnation of the fibers by the adjacent thermoplastic molten resin. The Dellavecchia procedure is carried out in a manner to prevent migration from one layer to the next, specifically the process is carried out to avoid migration of the long reinforcing fibers to the outer resinous layer and also to avoid migration of the short fibers, if present in the outer resin layer, into the reinforcing layer.